Scan induction heating

ABSTRACT

An apparatus and process are provided for scan induction heating of a workpiece. The workpiece is moved through an inductor to inductively heat treat features of the workpiece with electric power of varying frequency and duty cycle to control the magnitude of electric power as the frequency changes. Alternatively the inductor is moved along the workpiece to inductively heat treat features of the workpiece with electric power of varying frequency and duty cycle to control the magnitude of electric power as the frequency changes, or a combination of simultaneous inductor and workpiece movement may be used.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application of application Ser. No. 11,261,097,filed Oct. 28, 2005, which application claims the benefit of U.S.Provisional Application No. 60/623,413, filed Oct. 30, 2004, both ofwhich applications are hereby incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

The present invention relates to induction heating of an elongatedworkpiece by scanning the workpiece with an induction coil.

BACKGROUND OF THE INVENTION

Elongated workpieces, such as a drive shaft, require heat treatment ofselected features on the workpiece. For example, a first feature, suchas a pinion gear, may be provided at one end of a drive shaft, and asecond feature, such as a universal coupling may be provided at theother end. The gear and coupling are of different physicalconfigurations and require different heat treatment patterns formetallurgical hardening of these components. Additionally a heat-treatedfeature may need to be tempered after heat treating to relievemetallurgical stresses in the material of the feature.

One method of heat treating the workpiece and features on the workpieceis by electric induction scanning (or progressive) heat treatment. Inthis process, the workpiece generally moves through one or more scaninductors, although in other arrangements, the workpiece may bestationery and the one or more scan inductors (coils) may move along thelength of the workpiece. AC power is applied to the scan inductor tocreate a magnetic field around the inductor. The field magneticallycouples with the workpiece to inductively heat the workpiece. AC powerto the scan inductor may be varied as the workpiece passes through theinductor. For example U.S. Pat. No. 3,743,808 teaches controlling theinduction power and/or the scanning velocity of the scan inductor bycomparing instantaneous power and the instantaneous velocity with aknown energy distribution profile. The rate at which the workpiece movesthrough the inductor (scan rate) can be used to control the degree ofheating at the cross section of the workpiece that is coupled with themagnetic field.

Induction heat depth of penetration (induced current depth ofpenetration, δ) of a workpiece can be calculated from the formula:

$\delta = {503\sqrt{\frac{\rho}{{µ\; F}\;}}}$

where δ is in meters; ρ is the electrical resistivity of the workpiecein ohm-meters; μ is the relative magnetic permeability of the workpiece;and F is the frequency of the supplied induction power in Hertz.Therefore depth of penetration is inversely proportional to the squareroot of the frequency of the applied current. If the workpiece has twofeatures with a first feature that requires heating to a shallow depthof penetration (e.g. 2.5 mm), and a second feature that requires heatingto a deeper depth of penetration (e.g. 4.5 mm), the conventional methoduses an inverter with a fixed output frequency, for example 10,000Hertz, to achieve the shallower depth of penetration. From the aboveequation, the inverter's output frequency should be lower than 10,000 Hzfor the deeper depth of penetration of the second feature of theworkpiece, but since the frequency is fixed, the induction heat scan ofthe second feature must be slowed down to allow for deeper heatpenetration by heat conduction into the second feature. Further becauseof the slower scan rate, inverter output power to the induction coilmust be reduced to avoid overheating of the surface of the secondfeature. Also a heat-treated feature may require tempering of theheat-treated feature to reduce stresses in the feature. Typically thefeature is first heat treated in a first scan with low power and fixedhigh frequency to heat treat to the required depth of penetration, andthen heated in a second scan with fixed low frequency to temper thefeature.

One object of the present invention is to vary the output frequency ofthe inverter while adjusting the output power level of the inverter bypulse width modulation, as required to inductively heat treat and/ortemper various features of a workpiece to different depths ofpenetration in an induction scan of the workpiece.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the present invention is an apparatus for, and method of,supplying ac power with varying frequency and duty cycle to a scaninduction coil based upon the heating requirements of the cross sectionof a workpiece moving through the scan coil. A position sensing means,such as a servomotor, can be used to provide an input to a processorthat compares the inputted instantaneous position of the workpiece witha stored table of workpiece position values, each of which workpieceposition values can be correlated with frequency, power level and timeduration that corresponds to the required applied heat energy at thatposition. In one embodiment of the invention, the processor utilizes analgorithm that outputs a pulse width modulation command to the switchinggate circuits of an inverter so that a decrease in the inverter'svoltage pulse width results in a lower output power from the inverter tooffset an increase in output power from the inverter at lowerfrequencies. Conversely an increase in the inverter's voltage pulsewidth results in a greater output power from the inverter to offset adecrease in output power from the inverter at higher frequencies.

Other aspects of the invention are set forth in this specification andthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing brief summary, as well as the following detaileddescription of the invention, is better understood when read inconjunction with the appended drawings. For the purpose of illustratingthe invention, there is shown in the drawings exemplary forms of theinvention that are presently preferred; however, the invention is notlimited to the specific arrangements and instrumentalities disclosed inthe following appended drawings:

FIG. 1 is a simplified diagrammatic view of one example of the scaninduction heating apparatus of the present invention;

FIG. 2 is a simplified schematic of one example of a power supply andload circuit used with the scan induction heating apparatus of thepresent invention;

FIG. 3( a) and FIG. 3( b) illustrate the application of pulse widthmodulation to change the inverter's output from full power to halfpower;

FIG. 4( a) illustrates the change in load current magnitude with achange in the frequency output of an inverter with no pulse widthmodulation;

FIG. 4( b) illustrates the change in load power magnitude with a changein the frequency output of an inverter with no pulse width modulation;

FIG. 4( c) illustrates the change in load resistance with a change inthe frequency output of an inverter with no pulse width modulation;

FIG. 4( d) illustrates the change in the Q factor of the load circuitwith a change in the frequency output of an inverter with no pulse widthmodulation;

FIG. 5( a) illustrates the relationship between an inverter's outputvoltage and load current with an inverter output frequency of 3,000Hertz and no pulse width modulation;

FIG. 5( b) illustrates the relationship between an inverter's outputvoltage and load current with an inverter output frequency of 10,000Hertz and no pulse width modulation;

FIG. 5( c) illustrates the relationship between an inverter's outputvoltage and load current with an inverter output frequency of 30,000Hertz and no pulse width modulation;

FIG. 6 illustrates the relationship between an inverter's output voltageand load current for an inverter using pulse width modulation in oneexample of the present invention; and

FIG. 7 is a simplified flow chart illustrating one example of theinduction power control scheme of the present invention for controllingscan induction power as the output frequency of the inverter is changedduring the scan.

DETAILED DESCRIPTION OF THE INVENTION

There is shown in the figures one example of the scan induction heatingapparatus of the present invention. In FIG. 1, inverter 10 suppliessingle phase ac power to scan inductor (coil) 12 via suitable electricalconductors such as bus bars. DC input to the inverter may be from anysuitable dc power source. The inductor may comprise any type of inductorknown in the art, and may be, for example, a single or multiple turninductor, or an assembly of individual inductors that are connected toone or more ac sources of power. Workpiece 14 is held in place by ameans for moving the workpiece through the inductor, which can be, forexample, a screw drive assembly 16, with extended arms, 16 a, to holdthe ends of the workpiece. Alternatively the workpiece may be stationaryand the inductor can be moved along the workpiece, or combined andcoordinated movement of both the workpiece and inductor can be used. Ameans for rotating the workpiece, such as electric motor 18, may also beprovided for rotating the workpiece as it moves through the inductor. Aposition sensing means, such as servomechanism 20, provides positionoutput signal 21 to processor 22. The position output signal indicatesthe Y-axis position of the cross section of the workpiece that is withinthe inductor (i.e. the section of the workpiece that is effectivelycoupled with the magnetic field generated by the flow of current in theinductor).

The workpiece can have one or more features, such as features 14 a, 14 band 14 c that may require different depths of current penetration ofinduction heating power for heat treatment and/or tempering as thosefeatures pass through the inductor. The regions of the workpiece betweenthese features may or may not require heat treatment. The multiplefeatures may be spaced apart as shown in FIG. 1 or located adjacent toeach other.

Processor 22 processes the output signal from the position sensing meansto determine the power level, frequency and time duration of inductionheating to be achieved at the inputted position of the workpiecerelative to the induction coil as further described below.

FIG. 2 is a simplified schematic of one example of an ac to dc powersupply used with inverter 10 that illustrates one method of supplying dcpower to the inverter. Rectifier section 30 comprises a full wave bridgerectifier 32 with ac power input on lines A, B and C supplied from asuitable source, such as utility power. Filter section 34 comprisescurrent limiting reactor L_(CLR) and dc filter capacitor C_(FIL).Inverter section 10 comprises four switching devices, S₁, S₂, S₃ and S₄,and associated anti-parallel diodes D₁, D₂, D₃ and D₄, respectively.Each switching device can be any suitable solid state device, such as aninsulated gate bipolar transistor (IGBT). The load circuit connected tothe output of inverter 10 comprises scan inductor L_(COIL) and workpiece14, which has regions, or features, that are coupled with the magneticfield generated around the inductor when the workpiece or inductor aremoved relative to each other. Resistance of the workpiece and the scaninductor (R_(COIL)) comprise the load resistance R_(LOAD).

FIG. 3( a) illustrates the typical output voltage waveform (FULLV_(OUT)) of the bridge inverter shown in FIG. 2 with no modulation ofthe voltage pulse width. Inverter switches S₁ and S₄ conduct during afirst time period, T₁, and inverter switches S₂ and S₃ conduct during anon-overlapping second time period, T₁, to produce the illustrated fulloutput voltage waveform with a frequency equal to ½T₁. FIG. 3( b)illustrates the typical output voltage waveform (HALF V_(OUT)) of thebridge inverter with 50 percent duty cycle (a). Each of the inverterswitches continues to conduct for the same period of time, T₁, as inFIG. 3( a), but with the conduction periods for switches S₃ and S₄advanced by half a time period (i.e., the duty cycle is equal to 50percent) to produce the illustrated half of full output voltage. Withthis arrangement, the load is shorted every half period. Changing theduration of the overlapping conduction periods for switches S₃ and S₄results in different values for the duty cycle. Since power isproportional to the square of the supplied voltage, the power applied tothe inductor will also change as the duty cycle changes. In the presentinvention variable frequency control is achieved by changing the timeperiod, T₁, while the magnitude of the voltage (power) is adjusted bychanging the duty cycle.

The effects on the output characteristics of a power supply with varyingoutput frequency that does not use the pulse width modulation control ofthe present invention is illustrated with a baseline load circuit for aparticular workpiece. For an inverter having output power of 100,000Watts (P(f₀)) at 635 volts (V_(OUT)), and frequency (f₀) of 10,000Hertz, baseline load circuit characteristics are established as:

L₀=30×10⁻⁶ Henries inductance of the inverter load;

R₀=0.4 ohms resistance of the inverter load; and

Q₀=(2·π·f₀·L₀)/R₀=4.712 for the load circuit Q factor.

Baseline peak load current, I₀, can be calculated as 772.45 amperes fromequation (1):

$I_{0} = {\frac{V_{OUT}}{R_{o}} \cdot {\left( {1 - {\mathbb{e}}^{\frac{- R_{0}}{2\;{L_{0} \cdot f_{0}}}}} \right).}}$

FIG. 4( a) illustrates the decrease in inductor current, I(f),normalized to the baseline current, as the output frequency, f, of theinverter increases, which can be calculated from equation (2):

${I(f)} = {\frac{V_{OUT}}{R_{o}\sqrt{\frac{f}{f_{o}}}} \cdot \left( {1 - {{\mathbb{e}}^{\frac{- R_{o}}{2\; L_{0}\sqrt{f \cdot f_{o}}})}.}} \right.}$

FIG. 4( b) illustrates the decrease in induction heating power, P(f),normalized to the baseline power, as the output frequency, f, of theinverter increases, which can be calculated from equation (3):

${P(f)} = {\frac{V_{OUT}^{2}}{2\; R_{o}\sqrt{\frac{f}{f_{o}}}} \cdot \left( {1 - {{\mathbb{e}}^{{\frac{- R_{o}}{2\; L_{0}\sqrt{f \cdot f_{o}}})}^{2}}.}} \right.}$

FIG. 4( c) illustrates the increase in load resistance, R(f), as theoutput frequency, f, of the inverter increases, which can be calculatedfrom equation (4):

${R(f)} = {R_{0} \cdot {\sqrt{\frac{f}{f_{0}}\;}.}}$

FIG. 4( d) illustrates the increase in the Q factor of the load circuitas the output frequency, f, of the inverter increases, which can becalculated from equation (5):

${Q(f)} = {Q_{0} \cdot {\sqrt{\frac{f}{f_{0}}}.}}$

FIG. 5( a) through FIG. 5( c) illustrate the generalized relationshipsin FIG. 4( a) through FIG. 4( d) for a specific example wherein pulsewidth modulation control of the present invention is not used. FIG. 5(c) graphically represents voltage and current outputs of an inverteroperating at rated full power and a frequency of 30,000 Hertz with nopulse width modulation control.

In FIG. 5( a) the output frequency of the inverter is lowered to 3,000Hertz and the current (and power) output is relatively high withoutpulse width modulation control. In the present invention pulse widthmodulation control of the inverter's output can be used to reduce thepower output of the inverter by using a relatively large duty cycle.

In FIG. 5( b) the output frequency of the inverter is at 10,000 Hertzand the power output is lower than the power output at 3,000 Hertzwithout pulse width modulation control, but still greater than the ratedfull power (current) of the inverter shown in FIG. 5( c). In the presentinvention pulse width modulation control of the inverter's output can beused with a lower duty cycle than that used at 3,000 Hertz to keep thepower output of the inverter at or below rated value.

In general, in the present invention, pulse width modulation control isused to change the inverter's output power at any operating frequencyfrom that which would occur without pulse width modulation control. Ingeneral duty cycle is decreased as frequency decreases to reduce theinverter's output power, and duty cycle is increased as frequencyincreases to increase the inverter's output power.

FIG. (6) further illustrates the characteristics of the load currentwith pulse width modulation control. When there is a non-zero inverteroutput voltage, the load current, I_(LOAD), can be calculated fromequation (6):

$I_{LOAD} = {\frac{V_{OUT}}{R_{LOAD}}{\left( {1 - {\mathbb{e}}^{\frac{R_{LOAD}}{L_{LOAD}} \cdot t}} \right).}}$

When there is zero inverter output voltage, the load current can becalculated from equation (7):

$I_{LOAD} = {I_{INTIAL} \cdot {\mathbb{e}}^{{- \frac{R_{LOAD}}{L_{LOAD}}} \cdot t}}$

where I_(INITIAL) is the magnitude of current when the inverter outputvoltage transitions to zero.

From FIG. 6, the shorter the duty cycle, the smaller the peak value ofthe load current (and power) before the load current drops when theoutput voltage is zero. Conversely the longer the duty cycle, the largerthe peak value of the load current (and power) before the load currentdrops when the output voltage is zero.

FIG. 7 illustrates a simplified flowchart for one non-limiting exampleof the scan induction heating process of the present invention. Theroutines identified in the flowchart can be implemented in computersoftware that can be executed with suitable hardware. Routine 100 inputsa workpiece (WP) scan coordinate (Y) that represents the position of theworkpiece within inductor 12. Routine 102 inputs values of power(P_(Y)), frequency (F_(Y)) and time (T_(Y)) for induction heating atposition Y. These values may be previously stored in a memory device,for example, as a lookup table based upon values established byexperimental testing of the workpiece with the apparatus. Alternativelyan operator of the scan induction apparatus may manually input thesevalues, or another method may be used to determine the requiredfrequency, power level, and, if used, the variable time value forinduction heat treatment of each position of the workpiece. Routine 104computes the required duty cycle (DC_(Y)) for the inverter output fromequation (8):Duty Cycle(in percent)=[P _(Y) /P(F _(Y))]×100,

where P(F_(Y)) is calculated from equation (3) with an appropriatebaseline load circuit determined from the actual workpiece beinginduction heat treated.

Routine 106 controls switching of the power supply's switching devicesto achieve the desired output frequency and duty cycle. In thisnon-limiting example, routine 106 outputs gate inverter control signalsto the gating circuits for the inverter's switches to achieve therequired frequency, F_(Y), and duty cycle, DC_(Y). Routine 108determines whether actual measured output power is at set power P_(Y).Actual measured output power may be inputted by use of suitable sensingdevices. If actual measured power is not equal to the required setpower, then the duty cycle is appropriately adjusted in routine 110, androutine 108 repeats. If actual measured power is equal to the requiredset power, then routine 112 checks to see if set time T_(Y) has expired.If the set time has not expired, then routine 108 is repeated; if settime has expired, then routine 114 outputs a control signal to theworkpiece's positioning system to advance the workpiece to the nextincremental position for induction heat treatment and returns to routine100 for execution. In other examples of the invention the time forinduction heating at each position Y will be the same for all positionsof the workpiece within the inductor; for this arrangement, frequencycontrol and duty cycle control, as frequency changes, are used toinduction heat each position of the inductor as each position is steppedthrough the inductor at a constant rate of speed.

In other examples of the invention, movement and positioning of theworkpiece through the inductor may be predetermined, for example, wherean induction scan apparatus sequentially heat treats many identicalworkpieces. In these arrangements, power, frequency, time, and dutycycle settings at each position of the workpiece may be predetermined byexperimental testing with the workpiece and the induction scan apparatusof the present invention, and executed without further inputting orcomputing any or all of these values for each successive identicalworkpiece heat treated with the apparatus. Incremental or sequentialpositioning of parts or features of the workpiece in the inductor can beaccomplished as discrete stepped movement of the workpiece or inductor,or a combination of both, either as fine, minute steps that approachcontinuous movement of the workpiece or inductor, or coarser stepsvisually discernable as stepped movement. While the terms “selectedpart,” “multiple features,” and “locations” are used to describesections of the workpiece placed within the inductor for induction heattreatment with variable frequency and duty cycle, the present inventionincludes varying the frequency and/or duty cycle while the part, featureor location passes through the inductor. That is subsections of eachpart, feature or location may be heat treated with varying frequenciesand duty cycles as the subsections of the part, feature or location passthrough the inductor.

In other examples of the invention pulse width modulation control can beused to control the inverter's power output as the output frequency ofthe inverter varies at a given workpiece position, for example, toachieve heat treatment and tempering for a feature of the workpiece.Further sequential heat treatment of features comprising the workpieceis not limited to sequential heat treatment in the order that thefeatures are positioned in the workpiece. For example, referring toworkpiece 14 in FIG. 1, features 14 a, 14 b and 14 c may be positionedand heat treated sequentially in that order through inductor 12.Alternatively, for example, features 14 a, 14 c and 14 b may bepositioned and heat treated sequentially in that order through theinductor.

It is noted that the foregoing examples have been provided merely forthe purpose of explanation and are in no way to be construed as limitingof the present invention. While the invention has been described withreference to various embodiments, it is understood that the words whichhave been used herein are words of description and illustration, ratherthan words of limitations. Further, although the invention has beendescribed herein with reference to particular means, materials andembodiments, the invention is not intended to be limited to theparticulars disclosed herein; rather, the invention extends to allfunctionally equivalent structures, methods and uses, such as are withinthe scope of the appended claims. Those skilled in the art, having thebenefit of the teachings of this specification, may effect numerousmodifications thereto and changes may be made without departing from thescope and spirit of the invention in its aspects.

The invention claimed is:
 1. An apparatus for induction heating of aworkpiece having a length, the apparatus comprising: a means for holdingthe workpiece stationary; a power source having an ac output with an acoutput frequency, the ac output having a duty cycle with pulse widthmodulation control; at least one inductor connected to the ac output togenerate an ac magnetic field, the at least one inductor at leastpartially surrounding the length of the workpiece; a means for movingthe at least one inductor to one or more inductor positions for a periodof time at each of the one or more inductor positions along the lengthof the workpiece to magnetically couple one or more selected parts ofthe workpiece with the ac magnetic field; a means for selectivelyadjusting the ac output frequency to a heat treatment frequencyresponsive to each one of the one or more inductor positions of the atleast one inductor along the length of the workpiece; and a means forselectively adjusting the power of the ac output by changing the dutycycle of the ac output to a heat treatment duty cycle responsive to theheat treatment frequency.
 2. The apparatus of claim 1 further comprisinga means for selectively adjusting the period of time that each of theone or more selected parts is coupled with the ac magnetic field forinduction heating.
 3. The apparatus of claim 1 further comprising asensor for sensing an actual power of the ac output and outputting theactual power to the means for selectively adjusting the power of the acoutput.
 4. The apparatus of claim 1 wherein the means for selectivelyadjusting the ac output frequency and power of the ac output comprises aprocessor having one or more output signals for control of the ac outputfrequency and voltage the bower of the ac output.
 5. The apparatus ofclaim 4 wherein the one or more output signals further comprises anoutput signal for controlling the means for moving the at least oneinductor responsive to the heat treatment frequency and the heattreatment duty cycle.
 6. A method of electric induction heat treatmentof a stationary workpiece having a length, the method comprising thesteps of: positioning at least one inductor at least partially aroundthe length of the stationary workpiece; moving the at least one inductorto a plurality of inductor positions along the length of the stationaryworkpiece; supplying an ac electric power to the at least one inductorto generate a magnetic field for coupling with a plurality of workpiecelocations on the length of the stationary workpiece for a period of timeat each one of the plurality of workpiece locations, each one of theplurality of workpiece locations corresponding to one of the pluralityof inductor positions, the ac electric power having a duty cycle, afrequency and a power magnitude; identifying a current one of theplurality of workpiece locations presently coupled with the magneticfield; adjusting the frequency of the ac electric power for the currentone of the plurality of workpiece locations; and adjusting the powermagnitude of the ac electric power by adjusting the duty cycle of the acelectric power when the frequency of the ac electric power is adjusted.7. The method of claim 6 further comprising the step of adjusting theperiod of time that the current one of the plurality of workpiecelocations is presently coupled with the magnetic field.
 8. The method ofclaim 7 further comprising the steps of adjusting the frequency of theac electric power for the current one of the plurality of workpiecelocations to a stored frequency value stored in a memory device for thecurrent one of the plurality of workpiece locations; adjusting the dutycycle of the ac electric power required for a stored power magnitude ofthe ac electric power stored in the memory device for the current one ofthe plurality of workpiece locations; and adjusting the period of timethat the current one of the plurality of workpiece locations is coupledwith the magnetic field to a stored time value stored in the memorydevice for the current one of the plurality of workpiece locations. 9.The method of claim 8 further comprising the step of calculating theduty cycle of the ac electric power by dividing the stored powermagnitude of the ac electric power by a computed value for the acelectric power with 100 percent duty cycle.
 10. The method of claim 8further comprising the steps of measuring an actual power magnitude ofthe ac electric power, comparing the actual power magnitude of the acelectric power with the stored power magnitude of the ac electric power,and further adjusting the duty cycle of the ac electric power toeliminate any difference between the actual power magnitude of the acelectric power and the stored power magnitude of the ac electric power.11. The method of claim 10 further comprising the steps of measuring anactual period of time that the current one of the plurality of workpiecelocations is coupled with the magnetic field, comparing the actualperiod of time to the stored time value, and advancing the at least oneinductor to another one of the plurality of workpiece locations when theactual period of time is equal to the stored time value.
 12. The methodof claim 6 further comprising the steps of adjusting the frequency ofthe ac electric power for the current one of the plurality of workpiecelocations to a stored frequency value stored in a memory device for thecurrent one of the plurality of workpiece locations, and adjusting theduty cycle of the ac electric power required for a stored powermagnitude of the ac electric power stored in the memory device for thecurrent one of the plurality of workpiece locations.
 13. The method ofclaim 12 further comprising the step of calculating the duty cycle ofthe ac electric power by dividing the stored power magnitude of the acelectric power by a computed value for the ac electric power with 100percent duty cycle.
 14. The method of claim 6 wherein the steps ofadjusting the frequency and adjusting the power magnitude of the acelectric power further comprise decreasing the duty cycle of the acelectric power when the frequency of the ac electric power decreases andincreasing the duty cycle of the ac electric power when the frequency ofthe ac electric power increases.
 15. A method of electric induction heattreatment of a stationary workpiece having a length, the methodcomprising the steps of: positioning at least one inductor at leastpartially around the length of the stationary workpiece; moving the atleast one inductor to a plurality of inductor positions along the lengthof the stationary workpiece; supplying a variable frequency electricpower to the at least one inductor to generate a magnetic field forcoupling with a plurality of workpiece locations on the length of thestationary workpiece for a period of time at each one of the pluralityof workpiece locations, each one of the plurality of workpiece locationscorresponding to one of the plurality of inductor positions, thevariable frequency electric power having a duty cycle, an outputfrequency and a power magnitude; identifying a current one of theplurality of workpiece locations presently coupled with the magneticfield; and adjusting the power magnitude of the variable frequencyelectric power when the output frequency of the variable frequencyelectric power changes by adjusting the duty cycle of the variablefrequency electric power responsive to the current one of the pluralityof workpiece locations.
 16. The method of claim 15 wherein the step ofadjusting the power magnitude of the variable frequency electric powercomprises decreasing the duty cycle of the variable frequency electricpower when the output frequency of the variable frequency electric powerdecreases and increasing the duty cycle of the variable frequencyelectric power when the output frequency of the variable frequencyelectric power increases.
 17. The method of claim 15 further comprisingthe step of varying the period of time at each one of the plurality ofworkpiece locations.
 18. The method of claim 15 further comprising thesteps of: correlating each one of the plurality of workpiece locationswith a memory stored frequency value and a memory stored electric powermagnitude value for the variable frequency electric power for each oneof the plurality of workpiece locations; adjusting the output frequencyof the variable frequency electric power for each one of the pluralityof workpiece locations to the memory stored frequency value for each oneof the plurality of workpiece locations; and adjusting the duty cycle ofthe variable frequency electric power so that the power magnitude of thevariable frequency electric power is equal to the memory stored electricpower magnitude value.
 19. The method of claim 18 further comprising thesteps of sensing an actual power magnitude of the variable frequencyelectric power supplied to the at least one inductor, and adjusting theduty cycle of the variable frequency electric power when the actualpower magnitude of the variable frequency electric power is not equal tothe memory stored electric power magnitude value.
 20. The method ofclaim 19 further comprising the step of adjusting the period of time ateach one of the plurality of workpiece locations to a memory storedperiod of time for each one of the plurality of workpiece locations. 21.Apparatus for induction heating of a workpiece having a length, theapparatus comprising: a power source having an ac output with an outputfrequency, the ac output having a duty cycle with pulse width modulationcontrol; at least one inductor connected to the ac output to generate anac magnetic field, the at least one inductor at least partiallysurrounding the length of the workpiece; a means for moving the at leastone inductor along the length of the workpiece to one or more inductorpositions to magnetically couple one or more selected parts of theworkpiece with the magnetic field, each of the one or more selectedparts corresponding to one of the one or more inductor positions; ameans for moving the workpiece through the at least one inductorsimultaneously with movement of the at least one inductor along thelength of the workpiece; a means for selectively adjusting the outputfrequency of the ac output to a heat treatment frequency responsive toeach one of the one or more inductor positions of the at least oneinductor; and a means for selectively adjusting the power of the acoutput by changing the duty cycle of the ac output to a heat treatmentduty cycle responsive to the heat treatment frequency.
 22. A method ofelectric induction heat treatment of a workpiece having a length, themethod comprising the steps of: positioning at least one inductor atleast partially around the length of the workpiece; simultaneouslymoving the workpiece and the at least one inductor to one or more heattreatment locations, each one of the one or more heat treatmentlocations corresponding to an inductor heat treatment position and aworkpiece heat treatment location; supplying an ac electric power withan output frequency to the at least one inductor to generate a magneticfield for coupling with the workpiece heat treatment location at each ofthe one or more heat treatment locations; identifying a currentworkpiece heat treatment location from the one or more workpiece heattreatment locations presently coupled with the magnetic field; adjustingthe output frequency of the ac electric power for the current workpieceheat treatment location; and adjusting the power magnitude of the acelectric power for the current workpiece heat treatment location byadjusting the duty cycle of the ac electric power when the outputfrequency of the ac electric power is adjusted.